O-bend lead for semiconductor packages



June 10, 1969 E @R1-NER ET Al. 3,449,642

o-BEND LEAD FOR sEMICoNDUcToR PACKAGES l Filed Feb. 27. 1968 UnitedStates Patent O U.S. Cl. 317-234 6 Claims ABSTRACT OF THE DISCLOSURE Alead for a semiconductor device has a central axial slot flattened toform a generally flattened opening through the lead which permitsflexing of the lead without transmitting excessive strain to thesemiconductor wafer to which it is connected.

This invention relates to semiconductor devices, and more particularlyrelates to a flexible lead structure for semi-conductor devices.

Semiconductor wafers such as thin monocrystalline silicon wafers usedfor semiconductor devices are brittle. Therefore, care must be taken toprevent application of excessive strains to the wafer from the leadsconnected thereto, due to thermal or mechanical conditions.

A commonly used lead connected to the wafer will have an expansion looptherein such as an S-shaped or a C-shaped bend so that the lead can flexdue to dimensional changes thereof during manufacture, assembly, and inuse without unduly straining the Wafer. Such leads require a certainminimum height for a given material and cross-section.

In accordance with the present invention, an expansion loop is providedwith a flattened O-shape, formed by a longitudinal slot in the leadwhich is compressed down to form two parallel arches, Since theexpansion loop is formed of two arches, the required height of the loopis considerably reduced in height for a given cross-section of a givenmaterial as compared to the standard S- and C-shaped expansion loops.Moreover, a standard lead material can be yused.

Another lead wire has been proposed in U.S. Patent 3,050,666 in which anopening is formed through a lead to permit yielding of the lead understress. This arrangement reduces the available cross-section of the leadfor current conduction, whereas the use of the flattened slot of theinvention retains the full available conduction area, even at the regionof flexing.

Accordingly, a primary object of this invention is to provide anexpansion loop in a lead for semiconductor devices which requires adecreased height.

A further object of tbs invention is to reduce the volume of asemiconductor device package.

Another object of this invention is to provide a novel expansion leadfor a semiconductor device which can be made of a variety of materials.

These and other objects of this invention will become apparent from thefollowing description when taken in connection with the drawings, inwhich:

FIGURE l shows a prior art semiconductor device with a lead having aC-shaped expansion loop.

FIGURE 2 is a cross-sectional view of FIGURE l taken across section line2-2 in FIGURE 1.

FIGURE 3 shows a prior art type S-shaped expansion loop.

FIGURE 4 shows a semiconductor device incorporating the flattenedOshaped extension loop of the invention.

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FIGURE 5 is a cross-sectional view of FIGURE 4 taken across section line5 5 in FIGURE 4.

FIGURE 6 shows a lead wire in a first stage of the manufacture thereofin accordance with the invention.

FIGURE 7 shows the lead of FIGURE 6 after flattening the slot therein.

Referring first to FIGURES 1 and 2, there is shown a semiconductor wafer10 secured to a conductive base 11 in any desired manner. Wafer 10 mayhave one or more junctions therein formed by diffusion, alloying, or thelike, to form the type of device desired, such a diode. By way ofexample, wafer 10 may be a square chip having a thickness of 17 mils andan area of 250 by 250 mils. The bottom of wafer 10 need not seat on base11, but may have any desired terminal secured thereto. The upperelectrode 12, connected to the top of wafer 10 is formed of a C-shapedportion 13 having a cross-section of about .013 square inch with aradius of about .045 inch, governed by the material thickness at thebend which dictates that an unfractured bend should have a radius of notless than the material thickness at the bend, and a corresponding heightabove the top of wafer 10 of about .180 inch. A lead 14 may be brazed tothe top of C-shaped portion 13. The bottom of C-shaped Iportion 13 isthen soldered or alloyed to the top of wafer 10. For example, lportion13 may be of aluminum to forrn an alloyed junction in wafer 10.

FIGURE 3 shows another prior art version of an expansion loop containinglead 15 having an S-shaped portion 16. 'Ihe cross-section of portion 16is similar to that of portion 13 of FIGURES 1 and 2, and will have aheight of .315 inch.

In accordance with the invention, and as shown in FIGURES 4 and 5, thelead 20 is formed in a novel manner, from a standard lead wire 21 whichmay be of silver, copper, aluminum, or any of the other usual lead wirematerials. In forming the lead, and as shown in FIGURE 6, a slot 22 isformed in wire 21, which may be a simple single cut for thin wirediameters, up to 188 mils, or a slot, shown in dotted lines 23 in FIGURE6, having a width of 30 to 60 mils for larger wire diameters. The lengthA of simple slot 22 is .180 inch for a wire diameter of 128 mils. Longerslots are Iused for larger diameter wires.

Wire 21 is then compressed axially to flatten slot 22 to the shape 24,shown in FIGURE 7, defined by two parallel arches 25 and 26. Dimension Bis about 45 mils, it being important only that the opposite surfaces ofopening 24 do not touch to permit exing in either direction. The wire 21is then cut off at about the dotted line 27, shown in FIGURE 7, with thelead extending above slot 24 for any desired length. The base of thelead is then soldered, or otherwise connected to the top of Wafer 10, asshown in FIGURES 4 and 5. Note that the expansion loop will have aheight H of about mils for a cross,- section equivalent to thecross-section of the lead of FIGURES l and 3. With this smaller height,it will be apparent that the size of the package for the semiconductordevice will be appreciably decreased.

As an alternate method of manufacture, a thin-walled, hollow metal tubecan be flattened to the shape of parallel arches 25 and 26 and a leadportion can be fastened by welding or brazing to the outer wall of thetube.

Although this invention has been described with respect to its preferredembodiments, it should be understood that many variations andmodifications will now be obvious to those skilled in the art, and it ispreferred, therefore, that the scope of the invention be limited not bythe specific disclosure herein, but only by the appended claims.

The embodiments of the invention in which an exclusive privilege orproperty is claimed are dened as follows:

1. A conductive lead wire for a semiconductor dcvice; said lead wirehaving an expansion loop therein; said expansion loop formed of aflattened slot in said wire, flattened in a plane perpendicular to theaxis of said wire; said flattened slot defining a pair of outwardlybowed arch Aportions of generally equal cross-section; the totalcross-section of said pair of arch portions being approximately equal tothe full cross-sectional area of said lead wire.

2. The lead wire of claim 1 wherein said lead wire has one at endsurface immediately below the bottom outer surface portions of said pairof outwardly bowed arches.

3. The lead wire of claim 1 wherein said lead wire has a circularcross-section.

4. The lead wire of claim 1 comprised of a flattened hollow metal tubedefining said bowed arch portions and a wire extending from andconnected to an outer surface portion of said tube.

5. In combination; a lead wire and a at semiconductorwafer; said leadwire having an expansion loop therein; said expansion loop formed of aflattened slot in said wire, flattened in a plane perpendicular to theaxis of said wire; said flattened slot defining a pair of outwardlybowed arch portions of generally equal cross-section; the

total cross-section of said pair of arch portions being approximatelyequal to the full cross-sectional area of said lead wire; said lead wirehaving one at end surface immediately below the bottom outer surfaceportions of said pair of outwardly bowed arches; said one end surfaceconnected to one surface of said wafer.

6. The method of forming an expansion loop in an electrical lead wirecomprising the steps of forming a slot through the center of a lead wireextending axially with said lead wire, and thereafter axiallycompressing said wire to bow the material thereof outwardly of oneanother to form a flattened slot extending perpendicularly of the axisof said wire with the opposite sides of said flattened slot spaced fromone another.

References Cited UNITED STATES PATENTS 2,651,009 9/1953 Meyer 3l72352,896,134 7/1959 Myer 317--234 3,050,666 8/1962 Stump 317-234 3,196,3257/1965 Swartz 317-234 JOHN W. HUCKERT, Primary Examiner.

S. BRODER, Assistant Examiner.

